This invention relates generally to magnetic flowmeters, and more particularly to a circuit for a bi-directional output type of magnetic flowmeter wherein the polarity of an input derived from a magnetic flowmeter is detected and inverted so that a constant polarity output signal can be produced at all times even though the flow direction in a flow tube alters.
In general, in a signal converter of a conventional magnetic flowmeter, a dividing circuit is provided. This circuit acts to obtain the ratio between a flow signal and a signal which is proportional to the excitation current of the flowmeter so as to eliminate measurement error caused by fluctuations in this current. The dividing circuit is usually composed of a multiplying circuit and a subtracting circuit. To simplify the construction thereof, the flow signal is first converted to a frequency signal by use of a voltage-to-frequency converter whose duty cycle is proportional to the flow signal. Then the output of the converter is applied to the multiplying circuit which is driven with the pulse signal.
The frequency of this pulse signal, which corresponds to a 0 to 100% volumetric flow rate and operates within a frequency band of, for example, 0 to 10 KHz, is varied with the output of a synchronous rectifier whose output changes in the negative (-) direction in accordance with a 0 to 100% input. As a result, when the flow direction is reversed and when the signal of the inverted polarity is applied to the synchronous rectifier, since the synchronous rectifier generates the inverted polarity output, the voltage-to-frequency converter does not respond to the output of the synchronous rectifier, and maintains a zero output. Thus the drawback of a conventional signal converter is that even though it may respond to an input derived from a fixed flow direction, it does not respond to the input derived from the other flow direction.
When the magnetic flowmeter is used, for example, for service water transactions, it is desirable to carry out both the receipt and delivery of the service water by an identical supply pipe, and to deal with service water transactions by means of a single flowmeter. To satisfy these requirements, bi-directional output-type flowmeters have heretofore been proposed.